Cofactors of Enzymes
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Names and structures of basic heterocycles. Coenzymes, cofactors, their functions, structures, examples of reactions, relation of cofactors to vitamins.
Cofactors of Oxidoreductases Nicotinamide Adenine Dinucleotide (NAD+)
conh2
1. Complete the remaining parts of the structure. What is the difference between NADP+ and NAD+?
2. NAD+ is composed of two nucleotides. Give their names and structures and describe how they are connected.
3. What is the function of both coenzymes?
4. Draw the structure of reduced form of NAD+. Why the reduced form of NAD+ has no positive charge?
5. What vitamin is a part of this coenzyme, what are its sources?
6. Write the equations of reactions of NAD+ with: a) lactate (catalyzed by lactate dehydrogenase)
b) ethanol (catalyzed by alcohol dehydrogenase).
NAD+-dependent enzymes e.g.   (citric acid cycle): ..... ..... .....
(complete) (glycolysis): ..... .....
(detoxification of ethanol): ..... .....
NADP+-dependent enzymes: e.g. (synthesis of fatty acids): oxoacyl reductase
(synthesis of cholesterol): HMG-CoA reductase
Describe the structure of both nucleotides.
Which heterocycles are contained in flavine part of this cofactor?
What is the name of structure depicted?
What is the function of FAD?
Which vitamin is a part of flavine cofactors?
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12. Write the structure of reduced form of FAD.
13. Write the equation of the reaction of FADFF with fumarate.
FMN-dependent enzymes: e.g. (respiratory chain): NADH dehydrogenase
FAD-dependent enzymes: e.g. (citric acid cycle): (complete) .....
(oxidative decarboxylation of 2-oxoacids): pyruvate dehydrogenase
14. Which heterocycles are comprised in the structure of tetrahydrobiopterin?
5,6,7,8-tetrahydrobiopterin
Function of BH4 in some reactions catalyzed by monooxygenases: r-h r-oh
nad+ nadh + h+
Enzymes utilizing BH4, e.g. (hydroxylases of aromatic acids):
(complete)
(synthesis of NO, see page 43): NO-synthase
Tetrahydrobiopterin (BH4)
O
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Coenzyme Q, Fe-S Proteins, Haem - see the chapter Respiratory chain, page 75. Molybdopterin (Molybdenum Cofactor)
HN
H
S—Mo
\
opo,2-
x= o, s
Enzymes utilizing molybdopterin, e.g.: xanthine oxidase, sulfite oxidase
Lipoic Acid
s—S
15. Complete the missing part of the structure
16. In which way is lipoic acid bonded to enzyme?
17. In which type of reaction is lipoic acid involved?
18. Draw the structure of dihydrolipoate.
Lipoate-dependent enzymes: (oxidative decarboxylation of 2-oxoacids): (complete)
Glutathione (GSH)
0
ii
H ?-■ —N-CH
CH2
SH
19. Complete the missing parts of the structure and their names.
20. GSH is involved in removal of H202 and organic peroxides in the cell. Write the common equation of such reaction.
21. How is reduced form of glutathione regenerated? Which coenzyme is necessary for it?
GSH dependent enzymes, e.g.  (selenoprotein): glutathione peroxidase
(not only oxidoreductases)     (conjugation of antibiotics): glutathiones-transferase
(membrane transport AA): y-glutamyl transpeptidase
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Cofactors of Transferases
Pyridoxal Phosphate
2-
ch20-p03
22. Which vitamin is related to pyridoxal phosphate?
23. Name the heterocycle that is contained in the pyridoxal phosphate structure.
24. What other forms of cofactor can be formed in the reaction with amino acids? Draw the partial structures.
Scheme of aldimine formation in reaction with amino acids
R-CH-COO" i + NHa
Transamination
i
R' PLP
Aldimine
(Schiffs base)
Decarboxylation
Pyridoxal phosphate dependent enzymes, e.g. (transamination): (complete)..... .....
(synthesis of haem): 8-aminolevulinate synthase
Biotin
O
A
hn nh
25. Which two heterocycles are contained in the molecule of biotin?
26. Which particular reaction is biotin involved in? Which group is transferred?
27. Which atom binds the group transferred?
Biotin-dependent enzymes, e.g. (anaplerotic reactions of citric acid cycle:.....
(complete) (synthesis of fatty acids): acetyl-CoA carboxylase
28. Which of the stated cofactors belongs among the coenzymes: FAD, NAD+, lipoate, biotin?
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Tetrahydrofolate (H4folate, FH4)
hn
h,n
"n l-l
10
,ch2—nh-
29. Describe the structure and complete the formula.
H4folate carries one-carbon fragments (complete):
Methyl .........
Methylene .........
Methenyl -CH=
Formyl .........
Formimine .........
attached to FH4 at
N5
N5,N'° N5, N'°
N5
Formation, interconversion and utilization of FH4 derivatives
Folate
1
FH,
I
FH4 -
Trp
I
Formiate
atp
FH4
His
*► Formimino-FHj
NH,
Formyl-FH4
Methenyl-FH4
Ser, Gly
FH4
Methylene-FH4
SAM <-
Met
► Hey
Purines
Methylene-FH4
reductase pj^o
dUMP
dTMP
Methyl-FH4
Hey ... homocysteine, SAM .. . S-adenosylmethionine
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30. Write the partial equation of the transformation of methenyl-FH, to methylene FH4 (NADH acts as cofactor).
31. Give the main sources of one-carbon units in the body.
32. State an example of a methylation reaction at which the methyl group is donated by FH4.
33. What is the principle of antibacterial action of sulfonamides?
Cobalamins (Vitamin B|2)
R ...
5'-deoxyadenosine (mitochondria) -OH (cytosol) -CH3 (cytosol)
34. Describe the cofactor structure.
35. Draw the structure of 5'-deoxyadenosine.
Significance of methylcobalamin
Methionine synthase
Hey
Met
Methylcobalamin
Hydroxocobalamin
Vitamin B|2 dependent enzymes, e.g. (methionine resynthesis): methionine synthase
(propionate utilization): methylmalonyl-CoA mutase
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36. Vitamin B|2 deficiency may lead to inactivation of FEL, ("folate trap"). Explain it.
37. A lack of vitamin B12 may result in disturbances in DNA synthesis preventing cell division, especially the formation of new erythrocytes. Consequently anaemia (megaloblastic or pernicious) is developed. Explain it.
Thiamine Diphosphate (TDP)
TDP dependent enzymes, e.g.   (citric acid cycle): .....
(oxidative decarboxylation of 2-oxo acids): ..... .....
transketolase reaction in pentose cycle
38. Give the names of both heterocycles in the molecule. Complete the formula.
39. What group is transferred and in which form?
40. Which atom of the cofactor binds the group transferred?
41. What vitamin is a part of cofactor structure?
42. What types of reactions TDP takes part in?
43. One of the symptoms of beriberi (in Sinhalese = / am not able to) is that blood pyruvate is markedly increased. Explain it. In which way the symptoms of this disease can be eliminated?
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